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15th-century developments

…but is now called the variation or declination. For a time, compass makers in northern countries mounted the needle askew on the card so that the fleur-de-lis indicated true north when the needle pointed to magnetic north. This practice died out about 1700 because it succeeded only for short voyages…

magnetic survey

…magnetic inclination, or dip, and declination (departure from geographic north) at several stations. If the object of the survey is to make a rapid reconnaissance of an area, a magnetic-intensity profile is made only over the target area. If the object of the survey is to delineate already discovered structures,…

use by Columbus

…discovered the principle of compass variation (the variation at any point on the Earth’s surface between the direction to magnetic and geographic north), for which he made brilliant allowance on the journey from Margarita Island to Hispaniola on the later leg of this voyage, and he also observed, though misunderstood,…

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